In known position measuring systems a scale having a graduation track thereon is scanned by a scanning unit which is displaceable relative to the scale. To measure an absolute position, several graduations arranged parallel to each other and having different resolution are provided. The scanning elements of the scanning unit read the graduations and generate electrical scanning signals of different periods which are linked with each other in an evaluation unit to determine the absolute position of the scanning unit with relation to the scale as is well known.
The scanning elements themselves, as well as the entire circuitry of the scanning elements, exhibit a frequency-dependent low-pass filter behavior. With increasing frequency of the scanning signals, this low-pass filter behavior causes increasing phase shifting and thus a phase error between the individual scanning signals of different frequency. The behavior and the properties of low-pass filters are described in the book by U. Tietze et al. entitled "Halbleiter-Schaltungstechnik" [Semiconductor Circuit Technology], 10th ed., pp. 9-13 and 143-155 (1993).
To compensate for the frequency-dependent phase shift of the individual scanning signals during position measurement it has been proposed to assign a summing time member to the scanning elements as described in European Patent Publication No. EP 0 256 229 B1. These summing time members are in the form of amplifiers, RC networks or all-pass circuits and are relatively expensive circuits with large space requirements. Since these circuits themselves also display frequency-dependent behavior, an exact optimization is difficult to realize and is only possible within a very limited frequency range. Furthermore, downstream-connected circuits with negative feedback tend to oscillate.
Thus, it is desirable to provide a position-measuring device and method in which the frequency-dependent effects on the scanning signals are compensated effectively and in a simple manner.